Perry Lassiter found an old GW in the archives in which I lay out what I am looking for in Good Words that bears repeating here:

I try to find words that we have not used before (look in the Good Word dictionary), and that tell a story about us and where we come from, how we really think, what we are currently up to, what our ancestors did, and our ancestors' opinion of what we are still doing.

I also like words that stand out because special properties: pronunciation problems, spelling peculiarities, accentuation issues. I am especially fond of "garden path" words, words that lead our semantic machine in the wrong direction, like fartlek.

Other words of the day only aim at vocabulary building—and most of our GWs are chosen for that purpose, too. However, the words of a language draw a map of the peoples who speak them and provide a map of each word's journey through a thousand doors. The more turns and twists that journey has (or is having) the better.